AlphaGo had won the first three matches.
In October 2015, AlphaGo became the first computer program ever to beat a professional Go player by winning 5-0 against the reigning 3-times European Champion Fan Hui (2-dan pro). That work was featured in a front cover article in the science journal Nature in January 2016.
AlphaGo made an error on move 79 and did not realize it until move 87.
Perhaps Lee Sedol is discovering a playing style that will cause AlphaGo significant problems or at least challenges.
Lee Sedol is playing brilliantly! #AlphaGo thought it was doing well, but got confused on move 87. We are in trouble now...— Demis Hassabis (@demishassabis) March 13, 2016